Many contactors of this type are capable of switching nominal current at say 100 Amps or 200 Amps, for a large number of switching load cycles, satisfactorily, the switching being done by suitable silver-alloy contacts containing certain additives, which prevent welding. The switching blades are configured to be easily actuated for the switching function, with minimal self heating at the nominal currents concerned.
Most meter specifications not only stipulate satisfactory Nominal-current Endurance switching—without the contacts welding—but also demand that at moderate short-circuit fault conditions they must also not weld, and must open on the next actuator-driven pulse. At much higher related “dead-short” conditions the switch contacts may weld, but must remain intact, not explode or emit any dangerous molten material during the “dead-short” duration, until protective fuses rupture, or circuit breakers drop-out and disconnect the mains supply to the load, safely. This shorting duration may be for a maximum of 6 cycles of the mains supply.
U.S. Pat. No. 7,833,034 introduced the basic configuration of the “bi-blade” switch comprising a pair of parallel movable spring-copper arms or blades, of a particular thickness, width and active length, with a small defined gap there between. The blades' fixed ends are terminated together by rivets, screws, or semi-shears, to a moving-blade-carrier terminal, with movable contacts attached on the inner faces of the free ends, which close naturally on fixed contacts attached to the other fixed-blade-carrier terminal of the switch.
In the basic embodiment, the contactor uses a bi-blade switch construction, in which the switch has a pair of movable arms (also known as blades), which are strip-punched and pre-formed so that they close on the fixed contacts with a defined “contact-pressure” force—for achieving a relatively low switch resistance—and the open ends are formed outwardly with a sloping portion. The arms extend parallel to each other and separated by a small gap so that under high current situations the currents through the arms create forces of magnetic attraction urging the arms towards each other and increasing the force applied to the fixed contacts disposed between the distal ends of the arms. This force of attraction offsets the repulsive force urging the contacts apart, and is also due to the high current passing through the contacts. This arrangement is shown in FIGS. 1 to 3. FIGS. 1 & 2 show a single-pole contactor 10 with the cover removed to show the workings. FIG. 3 is a schematic view of the arms 30 of one switch. Each arm has a strip of spring copper having a first end 34 attached to a first terminal 24, known as the movable terminal as it is connected to the movable arms. A second terminal 22, known as the fixed terminal has fixed contacts 23. The distal end 36 of each arm is fitted with a movable contact 25. Each arm 30 has a sloping section or portion 38 to create an offset between the ends of the arms such that the fixed contacts can be accommodated between the movable contacts. The two arms extend parallel to each other except at the sloping portion. The movable contacts are arranged to align with the fixed contacts and in the relaxed state of the arms, the movable contacts bear against the fixed contacts with a predetermined contact force. The arms are able to move or flex within the plane of the drawing about the connection to the first terminal. A rib 39 is formed in the arms to stiffen the arms against excessive flexing.
The basic parallel “bi-blade” configuration, as used in a 100 Amp nominal current contactor, creates dynamic magnetic blade forces in excess of the contact repulsion forces during short-circuit faults. The blade geometries and contacts were optimised to avoid welding at the specified operating conditions. This basic 100 Amp switch uses 4 contacts; two movable and two fixed, with 50 Amps in each parallel blade. This basic arrangement was not capable of withstanding much higher nominal and short-circuit currents, as the blade geometries and current-sharing parameters limited the balancing of the blade forces and particularly the greater contact repulsion forces, resulting in much lessened endurance life, and serious contact welding issues during higher short-circuit faults.
U.S. Pat. No. 7,833,034 also introduced the divided blade concept, allowing a 200 Amp nominal current contactor able to balance the dynamic magnetic blade forces and contact repulsion forces during short-circuit faults, the geometries and contacts being optimised to avoid welding at the specified conditions.
To evenly share the current sharing—and to balance the repulsive contact forces and blade magnetic attraction forces—each adjacent parallel “bi-blade” was sub-divided into longitudinal half-blades, with a movable contact at each of their free ends, mating with respective fixed contacts, thus constituting 4 half-blades in parallel with 8 contacts per switch, or 16 in total for the 2-pole, two-phase disconnect contactor. This lower current sharing in each half-blade significantly reduces the contact repulsion forces.
Thus at 200 Amps, each half-blade will be carrying only 50 Amps, reducing the burden per half-blade when switched, minimising self heating, and avoiding welding at the higher nominal and short-circuit currents. Importantly, all half-blade currents flow in the same direction, thus maximising the magnetic attraction forces between half-blades in the working gap, especially at high current, to keep the contacts tightly closed.
The existing 100 Amp switch designs using simple parallel spring-copper “bi-blades” are very limited by the geometries and gap between, each blade in the “bi-blade” set being capable of generating certain magnetic attraction forces at high shared current, one with-respect-to the other, balanced and acting against the contact repulsion forces—both being proportional to the square of the current—in order to ensure that the contacts remain closed during short-circuit faults. It is very difficult to get this balanced ratio of forces exactly right for a particular configuration. Hence the divided blade version was optimised for use at 200 Amps, but used longer blades and 16 contacts in total.
The divided bi-blade configuration provided a good solution for the 200 Amp contactor but at a price as the silver contacts are expensive and the divided blades take up space. There is also a market want for the 100 Amp and 200 Amp contactors to be made smaller to save space. Thus there is a desire to reconfigure the simpler, basic parallel “bi-blade” 100 Amp switch geometry and configuration, so it was capable to operate at the higher 200 Amps nominal current with a greater short-circuit capability, in full compliance with various National requirements such as the ANSI C12.1 meter-disconnect specification.
Certain embodiments of the present invention provide a smaller, simpler, cost-reduced switch, using a new “bi-blade” switch arrangement, which not only uses less copper blade material, but requires only 8 switching contacts per 2-pole contactor instead of the current 16 required in the present design for a contactor rated at 200 Amps nominal current. Silver-alloy contacts represent a significant proportion of all high-current contactor cost breakdowns, so a reduction in the number of contacts required for a particular switching function is a major cost-saving benefit. Teachings from the improvements to the 200 Amp contactor can be applied to contactors rated at 100 Amps or less, to reduce its size.